Fill tube with vitreous coating

ABSTRACT

A fill tube for die casting machine includes a vitreous coating thereon effective to reduce or eliminate the flow of air through the gas permeable wall of the fill tube during die casting when air pressure is applied on the molten metal or alloy in the vessel.

FIELD OF THE INVENTION

[0001] The present invention relates to a fill tube for a die casting machine.

BACKGROUND OF THE INVENTION

[0002] In certain die casting machines, the molten metal or alloy to be cast is held in a vessel. A fill tube extends from below the surface of the molten metal or alloy to a die cavity. When gas (e.g. air) pressure is applied for a time to the molten metal or alloy in the vessel, the molten metal or alloy is forced through the fill tube into the die cavity in an amount to fill the die cavity. In an automated die casting machine, gas pressurization on the molten metal or alloy in the vessel is controlled to periodically introduce charges of the molten metal or alloy from the vessel into the die cavity.

[0003] Automated die casting machines have employed a gas permeable ceramic fill tube past to convey the molten metal or alloy from the vessel to the dies forming the die cavity therebetween. Use of a gas permeable ceramic fill tube is disadvantageous since pressurized ambient air (or other pressurizing gas) can pass through the gas permeable wall of the fill tube to an extent that the pressure differential necessary to force the molten metal or alloy through the fill tube to the die cavity is not achievable. Moreover, pressurizing gas passing through the wall of the fill tube can be conveyed to the die cavity where the air (gas) can be incorporated in the molten metal or alloy and as a result cause defects in the casting solidified in the die cavity.

SUMMARY OF THE INVENTION

[0004] The present invention overcomes this disadvantage by providing a vitreous coating on the ceramic fill tube wherein the vitreous coating is effective to reduce or eliminate the flow of air through the gas permeable wall of the fill tube during die casting when air pressure is applied on the molten metal or alloy in the vessel.

[0005] In an illustrative embodiment of the invention, the vitreous coating is made by mixing appropriate proportions of glass frit and deionized water and coating a surface of the ceramic fill tube with the mixture. The coating on the fill tube then is fired to produce the vitreous coating that reduces or eliminates the flow of air through the gas permeable wall of the fill tube during die casting when air pressure is applied on the molten metal or alloy in the vessel.

[0006] Advantages of the present invention will become more readily from the following detailed description of the invention taken with the following drawings.

DESCRIPTION OF THE DRAWINGS

[0007]FIG. 1 is a schematic sectional view of a fill tube disposed between a holding vessel for molten metal or alloy and a pair of dies forming a die cavity therebetween.

[0008]FIG. 2 is an enlarged sectional view of the fill tube showing the vitreous coating on the exterior surface of the fill tube.

DESCRIPTION OF THE INVENTION

[0009] Referring to FIG. 1, the present invention provides a fill tube 10 for use in a die casting machine. One end 10 a of the fill tube 10 is immersed in the molten metal or alloy M in a melt holding vessel 20, while the other end 10 b is communicated to a die cavity 30 formed between dies 32, 34 via a passage 36 in die 32. The fill tube 10 includes an interior passage 10 c along its length for conveying molten metal or alloy between melt holding vessel 20, such as a crucible, and the die cavity 30. A gas pressurizable chamber C is disposed above the molten metal or alloy in vessel 20 and can be gas pressurized to an extent to apply gas pressure on the molten metal or alloy M in the vessel 20 and force it upwardly through the fill tube 10 into the die cavity 30. For purposes of illustration and not limitation, the gas pressure can comprise air pressure provided to the chamber C by a conventional air compressor or pump 40 via a conduit 42 between the compressor and the chamber. In operation of the die casting machine, the timing of gas pressurization of the vessel 20 is controlled in a manner to periodically pressurize the molten metal or alloy in the vessel so as to introduce successive charges of the molten metal or alloy from the vessel into the die cavity 30 as is well known.

[0010] In the low pressure die casting aluminum and its alloys, the ceramic fill tube 10 can comprise zirconia or other suitable ceramic material resistant to the molten aluminum and its alloys. Other ceramic materials can be selected for the fill tube 10 depending upon the molten metal or alloy being die cast. The fill tube 10 typically is a preformed monolithic tube having appropriate shape and dimension for the die casting operation being conducted.

[0011] Pursuant to an illustrative embodiment of the present invention, the ceramic fill tube 10 is coated in-situ with a vitreous coating 11 as shown in FIGS. 1 and 2. The vitreous coating 11 is selected to exhibit reduced gas permeability as compared to the fill tube 10 so as to reduce or eliminate the passage or infiltration of a pressurizing gas in the vessel 10 through the gas permeable wall 10 w into the passage 10 c of the fill tube 10 during die casting when gas pressure is applied on the molten metal or alloy in the vessel 10. Preferably, the vitreous coating 11 is substantially gas impermeable to the pressurizing gas in chamber 35 to this end.

[0012] In an illustrative embodiment of the invention, the vitreous coating is formed in situ on the fill tube by mixing appropriate proportions of a glass frit and a carrier agent, which preferably comprises deionized water, to form a glass-forming mixture and coating the exterior surface of the ceramic fill tube 10 with the mixture. Other carrier agents, such as for example colloidal silica, may be used in practice of the invention depending upon the ceramic selected for the fill tube 10. The exterior surface of the fill tube 10 can be coated by dipping the fill tube 10 (with open ends plugged closed) in the glass-forming mixture or, alternately, by brushing, spraying, or otherwise applying the glass-forming mixture on the exterior surface of the fill tube 10. In general, the exterior and/or the interior surface of fill tube 10 can be coated with the same or different vitreous coating. The vitreous coating(s) should be compatible with the molten metal or alloy being cast through the fill tube so as not to be adversely affected by the particular molten metal or alloy being cast. The coating of the glass-forming mixture on the fill tube then is fired at a suitable firing temperature to produce the vitreous coating 11 that reduces or eliminates the passage of gas through the gas permeable wall 10 b into fill tube passage 10 a during die casting when gas pressure is applied on the molten metal or alloy in the melt holding vessel. 20.

[0013] For purposes of illustration and not limitation, a glass-forming mixture suitable for use with a zirconia fill tube used in a low pressure aluminum die cast machine can be made by mixing 4 parts by weight of a commercially available glass frit to 6 parts by weight of deionized water. A suitable glass frit can be obtained by mixing equal parts by weight of Ferro 3225 frit and Ferro frit 3226 from the Ferro Corporation. The resulting mixture has a chemistry of approximately 58.75% SiO₂, 29.10% B₂O₃, 4.60% Al₂O₃, 3.75% MgO, 2.20% Na₂O, and 1.60% CaO where %'s are by weight. The glass frit and deionized water can be mixed by first weighing out appropriate amounts of each constituent, placing the constituents in a container, and shaking the container to disperse the glass frit in the deionized water to form the glass-forming mixture or material. This glass-forming mixture or material then is applied to the fill tube 10 by dipping the fill tube therein with the open ends of the fill tube 10 plugged closed so that the interior surface of the fill tube are not coated with the mixture. The fill tube is held in the glass-forming mixture or material for a period of time (e.g. 1 second) and then removed and air dried before further handling. The dried coated fill tube 10 then transferred to a kiln and fired between 850 to 1000 degrees C. in air to consolidate the glass frit into a substantially gas impermeable vitreous coating 11 formed in-situ on the exterior surface of the fill tube 10. For purposes of illustration and not limitation, a dried coated zirconia fill tube can be heated to 980 degrees C. by ramping up at 1 degree C./hour, holding at temperature for 1 hour, and ramping down to room temperature at 1 degree C./hour. The fired vitreous coating 11 typically has a thickness of 0.001 to 0.004 inch, although other coating thicknesses can be used in practicing the invention

[0014] In practicing the invention, other glass frits can be used that are selected to be compatible with the particular ceramic material from which the fill tube 10 is formed. The thermal behavior (e.g. thermal expansion coefficient) of the vitreous coating and fill tube should be compatible to avoid to cracking, flaking off or other damage to the coating during service of the fill tube 10 for die casting. Moreover, other coating techniques for applying the glass-forming mixture on the fill tube and other firing temperatures and times can be used depending upon the glass-forming mixture employed.

[0015] Referring to FIG. 1, the fill tube 10 having the vitreous coating 11 thereon is disposed between the holding vessel 20 and the dies 32, 34 in conventional manner to convey the molten metal or alloy in the vessel 20 to the die cavity 30 when the chamber C is gas pressurized. For example, in the low pressure die casting of aluminum and its alloys, the chamber C typically is periodically pressurized for a predetermined time with air to for example 11 psi or other superambient pressure to force a desired charge of molten aluminum or an alloy thereof through the fill tube to the die cavity. The vitreous coating 11 on the exterior and/or interior surface of fill tube 10 reduces or eliminates the passage of the pressurizing gas (e.g. air) in the chamber C through the wall 10 w into the passage 10 c of the fill tube 10 to thereby provide the necessary pressure differential to force the charge of the molten aluminum or alloy thereof into the die cavity 30. The vitreous coating 11 also reduces or prevents the pressurizing gas from entering the passage 10 c and being conveyed to the die cavity 30, thereby reducing or eliminating casting defects attributable to gas (air) in the die cavity 30 and/or casting solidified in the die cavity 30.

[0016] Although the invention has been described above with respect to certain embodiments, those skilled in the art will appreciate that the invention is not limited to these embodiments since modifications, changes, and the like can be made therein without departing form the spirit and scope of the invention as set forth in the appended claims. 

I claim:
 1. A ceramic fill tube for a die casting machine wherein said fill tube includes a passage for conveying molten metal or alloy, said fill tube having a vitreous coating that reduces passage of a pressurizing gas present outside of said fill tube into said passage.
 2. The fill tube of claim 1 wherein said vitreous coating is substantially gas impermeable.
 3. The fill tube of claim 1 wherein said vitreous coating has a thickness of 0.001 to 0.004 inch.
 4. The fill tube of claim 1 comprising a zirconia tube.
 5. A die casting machine having a vessel for holding a molten metal or alloy, a die cavity, and a ceramic fill tube disposed between said die cavity and said vessel and having a passage for conveying the molten metal or alloy from said vessel to said die cavity, said fill tube having a vitreous coating that reduces passage of a pressurizing gas present outside of said fill tube into said passage.
 6. The machine of claim 5 wherein said vitreous coating is substantially gas impermeable.
 7. The machine of claim 5 wherein said vitreous coating has a thickness of 0.001 to 0.004 inch.
 8. The machine of claim 5 wherein said fill tube comprises a zirconia tube.
 9. A method of making a ceramic fill tube for a die casting machine wherein said fill tube includes a passage for conveying molten metal or alloy, comprising applying a glass-forming material on said fill tube and firing said fill tube at an elevated temperature to convert said material to a vitreous coating in-situ on said fill tube such that said vitreous coating reduces passage of a pressurizing gas present outside of said fill tube into said passage.
 10. The method of claim 9 wherein said fill tube is dipped in said glass-forming material.
 11. The method of claim 9 wherein after firing, said vitreous coating is substantially gas impermeable.
 12. The method of claim 9 wherein said vitreous coating has a thickness of 0.001 to 0.004 inch.
 13. The method of claim 9 wherein said fill tube comprises a zirconia tube.
 14. A method of die casting a melt comprising aluminum, comprising providing a pressurizing gas on said melt to force it to flow through a passage in a fill tube wherein said fill tube has a vitreous coating that reduces passage of said pressurizing gas present outside of said fill tube into said passage.
 15. The method of claim 14 including gas pressurizing a chamber above said melt.
 16. The method of claim 15 wherein said chamber is pressurized with air.
 17. The method of claim 14 wherein an end of said fill tube is immersed in said melt and another end is communicated to said die cavity.
 18. The method of claim 14 wherein said vitreous coating is provided on an exterior and/or interior surface of said fill tube. 